Color imaging method employing a monolayer of beads

ABSTRACT

A polychromatic imaging system, the imaging member generally comprising a mosaic pattern having a multiplicity of a plurality of contiguous discrete small areas of different color, on a substrate, each discrete area containing electrically photosensitive particles of a single color in a matrix of softenable material; the process steps generally comprising in a preferred embodiment, uniformly electrostatically charging the member, exposing it to a polychromatic radiation pattern and developing to cause migration of the photosensitive particles to the substrate in colored configuration corresponding to said polychromatic radiation pattern. Typically, subtractive color images may be produced where the areas include cyan-colored particles sensitive mainly to red light, magenta-colored areas sensitive mainly to green light and yellow-colored particles sensitive mainly to blue light. Novel imaging members and methods of making same are also described.

United States Patent [191 Goffe et a1.

[ COLOR IMAGING METHOD EMPLOYING A MONOLAYER 0F BEADS [75] Inventors: William L. Golfe, Webster; Robert W. Gundlach, Victor, both of NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

22 Filed: Aug. 29, 1974 21 Appl. No.: 501,571

Related 1.1.8. Application Data [63] Continuation of Ser. NO. 281,072, Aug. 16, 1972, abandoned, which is a continuation of Ser. No. 679,774, Nov. 1, 1967, abandoned.

3/1973 Goffe 96/l.2 3/1974 Gundlach 96/1 PS Primary ExaminerNorman G. Torchin Assistant Examiner.lohn R. Miller ABSTRACT A polychromatic imaging system, the imaging member generally comprising a mosaic pattern having a multiplicity of a plurality of contiguous discrete small areas of different color, on a substrate, each discrete area containing electrically photosensitive particles of a single color in a matrix of softenable material; the process steps generally comprising in a preferred embodiment, uniformly electrostatically charging the member, exposing it to a polychromatic radiation pattern and developing to cause migration of the photosensitive particles to the substrate in colored configuration corresponding to said polychromatic radiation pattern. Typically, subtractive color images may be produced where the areas include cyan-colored particles sensitive mainly to red light, magenta-colored areas sensitive mainly to green light and yellow-colored particles sensitive mainly to blue light. Novel imaging members and methods of making same are also described.

27 Claims, 6 Drawing Figures US. Patent Oct.14,1975 SheetlofZ 3,912,505

FIG?

CTMIYICIMIYICIMIYICIMIYICIMIYICIMIYICTMNICNLYJQJMDL m INVENTOR. WILLIAM L. GOFFE ROBERT W. GUNDLACH 9mm c.. 9.12,

A 7' TORNE VS US. Patent Oct. 14, 1975 Sheet 2 of2 3,912,505

FIGS

ML MLQMQlsHLQIL l l n I INVENTOR.

WILLIAM L. GOFFE BY ROBERT W. GUNDLACH CL. Pm,

- ATTORNEYS COLOR IMAGING METHOD EMPLOYING A MONOLAYER OF BEADS CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation of copending application Ser. No. 281,072, filed Aug. 16, 1972, now abandoned, which is a continuation of application Ser. No. 679,774, filed Nov. 1, 1967, now abandoned.

BACKGROUND OF THE INVENTION This invention relates in general to imaging, and more specifically to a new polychromatic migration imaging system.

There has recently been developed a migration imaging system capable of producing high quality images of high density, continuous tone, and high resolution, an embodiment of which is described in copending application Ser. No. 460,377, filed June 1, 1965, now U.S. Pat. No. 3,520,681. Generally according to an embodiment thereof, an imaging member comprising a conducting substrate with a layer of softenable (herein also intended to include soluble) material containing photosensitive particles overlaying the conductive substrate is imaged in the following manner: a latent image is formed on the member, for example, by uniformly electrostatically charging and exposing it to a pattern of activating electromagnetic radiation. The imaging member is then developed by exposing it to a solvent which dissolves only the softenable layer. The photosensitive particles which have been exposed to radiation migrate through the softenable layer as it is softened and dissolved, leaving an image of migrated particles corresponding to the radiation pattern of an original, on the conductive substrate. The image may then be fixed to the substrate. Those portions of the photosensitive material which do not migrate to the conductive substrate may be washed away by the solvent with the softenable layer. As disclosed therein, by other developing techniques, the softenable layer may at least partially remain behind on the substrate.

In general, three basic imaging members may be used: a layered configuration which comprises a sub strate coated with a layer of softenable material, and a fracturable and preferably particulate layer of photosensitive material at or embedded near the upper surface of the softenable layer; a binder structure in which the photosensitive particles are dispersed in the softenable layer which overcoats a substrate; and an overcoated structure in which a substrate is overcoated with a layer of softenable material followed by an overlayering of photosensitive particles and a second overcoating of softenable material which sandwiches the photosensitive particles. Fracturable layer or material as used herein, is intended to mean any layer or material which is capable of breaking up during development and permitting portions to migrate towards the substrate in image configuration.

This imaging system generally comprises a combination of process steps which includes forming a latent image and developing with a solvent liquid or vapor, or heat or combinations thereofto render the latent image visible. In certain methods of forming the latent image, non-photosensitive or inert, fracturable and particulate layers and material may be used to form images as de scribed in copending application Ser. No. 483,675, filed Aug. 30, 1965, now U.S. Pat. No. 3,656,990

wherein a latent image is formed by a wide variety of methods including charging in image configuration through the use of a mask or stencil or first forming such a charge pattern on a separate photoconductive insulating layer according to conventional xerographic reproduction techniques and then transferring this charge pattern to the members hereof by bringing the two layers into very close proximity and utilizing breakdown techniques as described, for example, in Carlson U.S. Pat. No. 2,982,647 and Walkup U.S. Pat. Nos. 2,825,814 and 2,937,943. In addition, charge patterns conforming to selected, shaped, electrodes or combinations of electrodes may be formed by the TESl" discharge techniques as more fully described in Schwertz U.S. Pat. Nos. 3,023,731 and 2,919,967 or by techniques described in Walkup Pat. Nos. 3,001,848 and 3,001,849, as well as by electron beam recording techniques, for example, as described in Glenn U.S. Pat. No. 3,113,179.

In another variation of this imaging system an image is formed by the selective disruption of a particulate material overlaying or in an electrostatically deformable, or wrinklable film or layer. This variation differs from the system described above in that the softenable layer is deformed in conjunction with a disruption of the particulate material as described more fully in copending application Ser. No. 520,423, filed Jan. 13, 1966, now abandoned.

The characteristics of the images produced by this new system are dependent on such process steps as charging, exposure, and development, as well as the particular combination of process steps. High density, continuous tone and high resolution are some of the image characteristics possible. The image is generally characterized as a fixed or unfixed particulate image with or without a portion of the softenable layer and unmigrated portions of the fracturable layer left on the imaged member, which can be used in a number of applications such as microfilm, hard copy, optical masks, and stripout applications using adhesive materials.

Recently, it has been discovered that polychromatic images may be prepared using a migration imaging system, as described above. Such a polychromatic system is described in copending application Ser. No. 609,056, filed Jan. 13, 1967, now abandoned. As described in that application, the fracturable layer of a layered configuration imaging member may comprise a mixture of different colored photosensitive particles, generally cyan, magenta and yellow for subtractive color imaging. An imaging member as described above, is prepared by overcoating a substrate with a softenable layer which is further overcoated with the mixture of different colored electrically photosensitive particles. The plate is electrostatically charged and exposed to a full color original. Typically. the resulting electrostatic latent image may be developed by dipping the plate in a solvent for the softenable layer. A subtractive color image conforming to the original is formed on the substrate with unneeded particles and the softenable layer being washed away by the solvent. This system is capable of producing good natural color images. However, it has been found that where a monolayer of particles is formed on the surface of the softenable layer, image density tends to be low. Where a thicker layer of particles is formed on the softenable layer surface, apparently some interference between particles of different color occurs giving images with less than desirable color balance and color separation.

Thus, while the above described polychromatic migration imaging system provides a simple and convenient method for preparing color copies, there is a continuing need for improvements which will give better color balance and better color density.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a polychromatic migration imaging system which overcomes the above-noted disadvantages and satisfies the above noted wants.

It is another object of this invention to provide a polychromatic migration imaging system capable of producing images of high color density.

It is another object of this invention to provide a polychromatic migration imaging system capable of producing images of excellent color balance.

It is another object of this invention to provide a material for producing polychromatic images which is highly resistant to degradation due to exposure to light, heat or high humidity.

Still another object of this invention is to provide a polychromatic imaging process of the lowest possible order of complexity.

Still another object of this invention is to provide a mosaic color imaging system utilizing a random mosaic pattern.

Still another object of the invention is to provide novel imaging members and methods of makingsame.

The foregoing objects and others are accomplished in accordance with this invention by providing a migration imaging member generally comprising a mosaic pattern having a multiplicity of a plurality of contiguous discrete small areas of different color, on a substrate, each discrete area containing electrically photosensitive particles of a single color in a matrix of softenable material; the process steps generally comprising in a preferred embodiment, uniformly electrostatically charging the member, exposing it to a polychromatic radiation pattern and developing to cause migration of the photosensitive particles to the substrate in colored configuration corresponding to said polychromatic radiation pattern. Typically, subtractive color images may be produced where the areas include cyan-colored particles sensitive mainly to red light, magenta-colored particles sensitive mainly to green light and yellowcolored particles sensitive mainly to blue light. Novel imaging members and methods of making same are also described.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following detailed disclosure of this invention taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a partially schematic view of the first step in one method of preparing the migration imaging member hereof.

FIG. 2 is a partially schematic illustration of a method of forming the bead layer of FIG. 1 into a smooth softenable layer containing photosensitive particles.

FIG. 3 is a partially schematic illustration of the uniform electrostatic charging step of the invention.

FIG. 4 is a partially schematic illustration of the step of exposing the charged softenable layer to polychromatic light.

FIG. 5 is a partially schematic illustration of an embodiment of the developing step of the invention; and,

FIG. 6 is a cross section of the imaging member of FIG. 3 after being processed according to the invention and comprising a polychromatic image on the substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown imaging member 10 according to this invention comprising substrate l1 and coated thereover layer 12 comprising beads, each bead containing photosensitive particles 13 of a single color, dispersed in beads of a softenable, electrically insulating material 14. The beads are of a plurality of colors and specifically comprise photosensitive magenta, yellow and cyan colored particles in a softenable material matrix.

Substrate 11 may be electrically conductive or insulating. Conductive substrates generally facilitate the charging or sensitization of the member according to the invention and typically may be of copper, brass, nickel, zinc, chromium, stainless steel, conductive plastics and rubbers, aluminum, steel, cadmium, silver, gold, or paper rendered conductive by the inclusion of a suitable chemical therein or through conditioning in a humid atmosphere to ensure the presence therein of sufficient water content to render the material conductive. The softenable layer may be coated directly onto the conductive substrate, or alternatively, the softenable layer may be self-supporting and may be brought into contact with a suitable substrate during imaging.

The substrate may be in any suitable form such as a metallic strip, sheet, plate, coil, cylinder, drum, endless belt, moebius strip or the like. If desired, the conductive substrate may be coated on an insulator such as paper, glass or plastic. Examples of this type of substrate are a substantially transparent tin oxide coated glass available under the trademark NESA from the Pittsburgh Plate Glass Co., aluminized polyester film the polyester film available under the trademark Mylar from DuPont, or Mylar coated with copper iodide.

Electrically insulating substrates may also be used which opens up a wide variety of film formable materials such as plastics for use as substrate 11.

Referring now to layer 12, the letters M, Y and C on each bead indicate that that bead contains magenta, yellow or cyan photosensitive particles, respectively. While in the embodiment shown in FIG. 1, the magenta, yellow and cyan particle containing beads are in a uniform pattern, in practice they would generally be in a random distribution. The photosensitive particles of each color respond to light of a different color so that the polychromatic image will be formed from a polychromatic original. Typically, an image may be formed on the above described plate by uniformly electrostatically charging the surface of the plate and exposing the plate to a polychromatic image. The softenable material matrix is then softened, as by dipping the plate in a solvent for the softenable layer. Particles which have been struck by light of an appropriate color migrate to the substrate leaving behind unexposed particles. An image is thus formed'on the substrate corresponding to a positive or a negative of the original, depending upon process variables.

In such a polychromatic system, the particles are selected so that those of different colors respond to different wavelengths in the visible spectrum corresponding to their principal absorption bands. The particles should be selected so that their spectral response curves do not have substantial overlap, thus allowing for color separation and subtractive multi-color image formation. In a typical multi-color system, the particle dispersion would include cyan colored particles mainly sensitive to red light, magenta colored particles sensitive mainly to green light and yellow colored particles sensitive mainly to blue light. These particles perform the dual function of final image colorant and photosensitive medium. When used in the imaging member described above, a subtractive color image is formed on the subtrate corresponding to the original. While three colors will be preferred for natural color imaging, two colors will be sufficient for many purposes, e.g., for forming printing on posters in two colors on a white or transparent background.

1 'Any suitable photosensitive particles 13 having the desired color characteristics may be used in carrying out the invention, regardless of whether the particular particle selected is organic, inorganic, is made up of one or more components in solid solution or dispersed one in the other or whether the particles are made up of multiple layers of different materials. Typical photosensitive particles include organic photoconductive insulating materials such as 8,13-dioxodinaphtho- (1,2,2',3)-furan-6-carbox-p-methoxyanilide; Locarno Red, C. I. No. 15865, l-(4-methyl-5- chloroazobenzene-Z'-sulfonic acid)-2-hydroxy-3- naphthoic acid; Watchung Red B, the barium salt of l- (4-methyl-5-chloroazobenzene-2-sulfonic acid)-2- hydroxy-3-naphthoic acid, C. I. No. 15865; Naphthol Red B, l-(2-methoxy-5-nitrophenylazo)-2-hydroxy- 3"-nitro-3-naphthanilide, C. 1. No. 12355; Duol Carmine, the calcium lake of l-(4'-methylazobenzene-2- sulfonic acid)-2-hydroxy-3-naphthoic acid, C. 1. No. 15850; Calcium Lithol Red, the calcium lake of 1-(2- azonaphthalene-l -sulfonic acid)-2-naphthol, C. I. No. 15630; Quinacridone and substituted quinacridones such as 2,9-dimethylquinacridone; Pyranthrones; lndofast Brilliant Scarlet Toner, 3,4,9,l0-bis(N,N-(pmethoxyphenyl)-imido)-perylene, C. 1. No. 71 140; dichloro thioindigo; Pyrazolone Red B Toner, C. I. No. 21120; phthalocyanines including substituted and unsubstituted metal and metal-free phthalocyanines such as copper phthalocyanine, magnesium phthalocyanine, metal-free phthalocyanine, polychloro substituted phthalocyanine, etc.; Methyl Violet, a phosphotungstomolybdic lake of a triphenylmethane dye, C. I. No. 42535; lndofast Violet Lake, dichloro-9.l8- isoviolanthrone, C. I. No. 60010; Diane Blue, 3,3- methoxy-4,4-diphenyl-bis(1-azo-2-hydroxy-3 naphthanilide, C. 1. No. 21180; lndanthrene Brilliant Orange R.K., 4,lO-dibromo-6,IZ-anthanthrone, C. I. No. 59300; Algol Yellow G.C., l,2,5,6-di(C,C'- diphenyl)-thiazole-anthraquinone, C. 1. No. 67300; Flavanthrone; lndofast Orange Toner, C. 1. No. 71 105; 1-cyano-2,3-phthaloyl-7,8-benzopyrrocoline and may other thio indigos, acetoacetic arylides, anthraquinones, perionones, perylenes, dioxazines, quinacridones, azos, diazos, thoazines, azines and the like; inorganics such as cadmium sulfide, cadmium sulfoselenide, zinc oxide, zinc sulfide. sulphur selenium. mercuric sulfide, lead oxide. lead sulfide, cadmium selenide, titanium dioxide, indium trioxide, and the like. In addition to the aforementioned pigments other organic materials which may be employed in the particles include polyvinylcarbazole; 2,4,-bis (4,4'-diethylaminophenyl l ,3,4-oxidiazole; N-isopropylcarbazole; polyvinylanthracene; triphenylpyrrol; imidazolidinone; 4,5-diphenylimidazolidinethione; 4,5-bis-(4'-aminophenyl )-imidazolidinone; l,2,5,6-tetraazacyclooctatetraene-(2,4,6,8); 3,4,-di-(4'-methoxyphenyl)-7,8- diphenyl-l ,2,5 ,6-tetraaza-cyclooctatetraene-( 2,4.6,8 3,4-di(4'-phenoxyphenyl )-7,8-diphenyl-l ,2,5,6- tetraaza-cyclooctatetraene-(2,4,6.8); 3,4.7.8- tetramethoxy-l ,2,5 ,6-tetraaza-cyclooctatetraene- (2,4,6,8); 2-mercapto-benzthiazole; 2-phenyl-4-alphanuphthylidene-oxazolone; 2-phenyl-4-diphenylideneoxazolone; 2-phenyl-4-p-methoxybenzylideneoxazolone; 6-hydroxy-2-phenyl(p-dimethyl-amino phenyl)-benzofurane; 6-hydroxy-2,3-di(p-methoxyphenyl)-benzofurane; 2,3,5,6-tetra(p-methoxyphenyl)- furo-(3,2f)-benzofurane; 4-dimethylaminobenzylidene-benzhydrazide; 4-dimethyl-aminobenzylideneisonicotinic acid hydrazide; turfurylidene (2)-4'-dimethylamino-benzhydrazide; S-benzilideneamino-acenaphthene3-benzylidene-aminc-carbazole; (4-N,N-dimethylamino-benzylidene)-p-N,N- dimethylaminoaniline; (Z-nitro-benzylidene)-p-bromoaniline; N,N-dimethyl-N 2-nitro-4-cyano-benzylidene )-p-phenylenediamine; 2,4-diphenylquinazoline; 2-(4'-amino-phenyl )-4-phenylquinazoline; 2--'pheny|-4-(4'-di-methyl-amino-phenyl) 7-methoxy-quinazoline; 1,3-diphenyltetrahydroimidazole; l,3-di-(4'-chlorophenyl)-tetrahydroimidazole; l,3-diphenyl-2-4'-dimethyl aminophenyl )-tetra-hydroimidazole; 1,3 ,-di-(p-tolyl )-2- lquinolyl-(20-)]-tetrahydroimidazole; 3-(4'-dimethylaminophenyl)-5-(4"-methoxy-phenyl)-6-phenyl-l,2,4-triazine; 3-pyridil-(4')-5-(4"- dimethylaminophenyl)-6-phenyl-l,2,4-triazine; 3-(4'- amino-phenyl)-5,6-di-phenyl-l,2,4-triazine; 2,5-bis[4'- amino-phenyl-( l )]-l.3,3-triazole; 2,5-bis[4-(N-ethyl- N-acetyl-amino)-phenyl-( l )]-l ,3,4-triazole; 1,5- diphenyl-3-methyl-pyrazoline; 1,3 ,4,5-tetraphenylpyrazoline; l-phenyl-3 -(p-methoxy styrl)-5-(pmethoxy-phenyl)-pyrazoline; l-methyl-2-(3,4'- dihydroxy-methylene-phenyl)-benzimidazole; 2-(4- dimethylamino phenyl)-benzoxazole; 2-(4'-methoxyphenyl)-benzthiazole; 2,5-bis-lp-amino-phenyl-( l 1,3,4-oxidiazole; 4,5-diphenylimidazolone; 3-aminocarbazole; copolymers and mixtures thereof.

Other materials which may be included in the particles include organic donor-acceptor (Lewis acid-Lewis base) charge transfer complexes made up of donors such as phenolaldehyde resins, phenoxies, epoxies, polycarbonates, urethanes, styrene or the like complexed with electron acceptors such as 2,4,7-trinitro-9 fluorenone; 2,4,5,7-tetranitro-9-fluo renone; picric acid; 1,3,5-trinitro benzene; chloranil; 2,5-dichlorobenzoquinone; anthraquinone-Z-carboxylic acid, 4- nitrophenol; maleic anhydride; metal halides of the metals and metalloids of groups l-B and ll-Vlll of the periodic table including for example, aluminum chloride, zinc chloride, ferric chloride, magnesium chloride, calcium iodide, strontium bromide, chromic bromide, arsenic triiodide, magnesium bromide, stannous chloride etc.; boron halides, such as boron trifluorides;

ketones such as benzophenone and anisil, mineral acids such as sulfuric acid; organic carboxylic acids such as acetic acid and maleic acid, succinic acid, citroconic acid, sulphonic acid, such as 4-toluene sulphonic acid and mixtures thereof.

Typical inorganic photoconductors include amorphous selenium; amorphous selenium alloyed with arsenic, tellurium, antimony or bismuth, etc.; amorphous selenium or its alloys doped with halogens; cadmium sulfide, zinc oxide, cadmium sulfoselenide, cadmium yellows such as Lemon Cadmium Yellow X-2273 from Imperial Color and Chemical Dept. of Hercules Powder Co., and many others. Middleton et al US. Pat. No. 3,121,006 lists typical inorganic photoconductive pigments.

As stated above, any suitable particle may be employed. The particles should be substantially insoluble in the matrix material and in the solvent used to develop the plate after exposure. Typical particles include those which are made up of only the pure photosensitive material or a sensitized form thereof, solid solutions or dispersions of the photosensitive material in a matrix such as thermoplastic or thermosetting resins, copolymers of photosensitive pigments and organic monomers, multi-layers of particles in which the photosensitive material is included in one of the layers and where other layers provide light filtering action in an outer layer or a fusible or solvent softenable core of resin or a core of liquid such as dye or other marking material or a core of one photosensitive material coated with an overlayer of another photosensitive material to achieve broadened spectral response. Other photosensitive structures include solutions dispersions, or copolymers of one photosensitive material in another with or without other photosensitively inert materials. Other particle structures which may be used, if desired, include those described in US. Pat. No. 2,940,847 to Kaprelian.

Softenable material 14 may be any suitable material which is soluble or softenable in a solvent liquid or vapor or heat or combinations thereof to permit selective migration of portions of the particles to the substrate, and in addition is substantially electrically insulating during the latent image forming and developing steps hereof. Where the softenable material 14 is to be dissolved away either during or after imaging it should be soluble in a solvent which does not attack the particles. Typical softenable materials include polyolefins such as polyethylene and polypropylene; vinyl and vinylidene resins such as polymethylmethacrylate and polyvinylcarbazole; polyamides; polyurethanes; polypeptides; polysulfides; polycarbonates; cellulosic polymers; polysulfones; phenolic resins; amino resins; epoxy resins; silicone resins; and mixtures and copolymers thereof.

Specifically, typical softenable materials include Staybelite Ester 10, a partially hydrogenated rosin ester, Foral Ester, a hydrogenated rosin triester, and Neolyne 23, an alkyd resin, all from Hercules Powder Co.; SR type silicone resins available from General Electric Corporation; Sucrose Benzoate, Eastman Chemical; Velsicol X-37, a polystyrene-olefin copolymer from Velsicol Chemical Corp.; Hydrogenated Piccopale 100, a highly branched polyolefin, Piccotex I polystyrene-vinyl toluene copolymer, Piccolastic A-75, 100 and 125, all polystyrenes, Piccodiene 2215, a polystyrene-olefin copolymer, all from Pennsylvania Industrial Chemical Corp.; Araldite 6060 and 6071, epoxy resins from Ciba; R506IA, a phenylmethyl silicone resin, from Dow Corning; Epon a bisphenol A- epichlohydrin epoxy resin, from Shell Chemical Corp.; PS-2, PS-3, both polystyrenes, ET693, a phenolformaldehyde resin, from Dow Chemical; a custom synthesized copolymer of styrene and hexylmethacrylate a custom synthesized polydiphenylsiloxane; a custom synthesized polyadipate; acrylic resins available under the trademark Acryloid from Rohm & Haas Co., and available under the trademark Lucite from Du- Pont; thermoplastic resins available under the trademark Pliolite from the Goodyear Tire & Rubber Co.; a chlorinated hydrocarbon available under the trademark Aroclar from Monsanto Chemical Co.; thermoplastic polyvinyl resins available under the trademark Vinylite from Union Carbide Co. and blends thereof.

The above group of materials is not intended to be limiting, but merely illustrative of materials suitable for softenable material 14.

Any suitable method of creating the mosaic pattern of discrete small areas of different color may be used. For example, the color areas may be printed by conventional multi-colored printing means such as gravure rollers or by conventional lithography. For example, three gravure rollers having registering groove patterns each containing an ink consisting of a softenable resin with photosensitive particles of a single color dispersed therethrough may be passed over the plate depositing a continuous pattern of alternating areas of different colors. For example, three half-tone screens may be prepared, one for each of the different color patterns so that when each plate is inked with a different color, patterns will be printed on the substrate in registration. Typical of the printing techniques which may be used are those described in Practical Photo-lithography" by C. M. Willy, Pittman and Sons, Ltd., London, 1952 and in Photography and Plate Making for Photolithography, l. H. Sayre, Lithographic Textbook Publishing Co., Chicago, I944. Also, the areas could be laid down by spraying the colored material through stencils.

However, an especially preferred method of forming the photosensitive particle containing softenable layer having a random arrangement of different colors is as follows. Small beads of the desired softenable material are formed having dispersed therethrough fine particles of the single color photosensitive particles. The beads would have a diameter slightly greater than the desired thickness for the softenable layer. Sets of beads having each of the desired colors are mixed together and spread on the substrate preferably as a monolayer. A smooth layer is then formed from these beads as by heating the beads to the melting temperature of the softenable material, for example, by passing a heated roller over the loaded beads as illustrated in H0. 2. Upon cooling, there is produced a smooth layer having discrete, contiguous areas containing photosensitive particles of different colors.

The beads may be formed by dispersing the photosensitive pigment in the softenable material and then forming beads by any conventional methods such as grinding or spray drying as described in copending application Ser. No. 380,080, filed July 2, 1964 now US. Pat. No. 3,338,991.

The beads of softenable material containing the photosensitive particles and the resulting smooth layer 16.

FIG. 2, formed therefrom, may have any suitable thickness. Generally, it is preferred that the layer have a thickness of from about V1 to about 16 microns. If the layer is thinner than about /2 micron, excessive background results upon development, while layers thicker than about 16 microns require relatively long development time resulting in lower image densities. Best results have been obtained with layers having a thickness of about I to 4 microns. Of course, the finer the mosaic pattern, whether it be formed by printing techniques or from beads of softenable material, the higher the resolution of the final image. Therefore, the range of about I to 4 microns in thickness for the final smooth layer is optimum.

The photosensitive particles may have any suitable diameter. Highest photosensitivity coupled with optimum color balance has been obtained with particles in the range of from about 0.02 to about 2 microns in size with optimum results for particles in the submicron range of from about 0.03 to about 0.5 microns.

The photosensitive particle containing beads may be spread across the substrate by any suitable, conventional method. While it is generally preferred that a monolayer of beads be formed on the substrate, it is possible to use multiple layers. One method of forming a uniform random monolayer of beads on a substrate is described by Vyverberg U.S. Pat. No. 2,759,450. In this method, the beads are electrostatically charged and blown in an air stream across the substrate. Due to electrostatic attraction forces, a substantially uniform layer of the beads is formed on the substrate. The layer of beads may be formed into a smooth layer by any suitable method.

Referring now to FIG. 2, there is shown one method of forming the bead layer into a uniform smooth softenable layer adhering to the substrate. A roller 18 internally heated as by steam line 20, to the softening temperature of the softenable bead material may be rolled across the plate surface, softening the beads and pressing them into firm contact with each other and the substrate. Often it is desirable to have a layer of a fluorinated hydrocarbon such as polytetrafluorethylene on the surface of roller 18 to prevent sticking of the beads to the roller. Boundary lines are shown in pressed and smoothed layer 16 between areas containing particles of different colors. These lines are primarily for purposes of clarity since in practice the resin will flow together eliminating apparent boundaries.

Other means may be used to form the beads into more uniform layers. For example, merely heating the plate to the softening temperature of the softenable material and allowing surface tension forces to smooth the layer is generally sufficient. The vapor of a solvent for the softenable material may also be used to soften the beads to create a smooth layer. Also, pressing the bead layer with a plate or roller under pressure without heat often is sufficient to form a smooth homogeneous layer.

It will be seen that the preferred method of forming the mosaic member hereof may also be used to form a binder structure, as described in copending application Ser. No. 634,757, filed Apr. 28, 1967, now abandoned wherein the photosensitive particle may be a single pigment of a single color.

Also, although the description herein speaks of depositing the beads on substrate 11, they may be deposited on a layer of softenable material which has previously been coated on the substrate. This technique permits the use of smaller diameter beads to achieve a desired total softenable layer thickness, which enhances resolution.

Also, an imaging member, as illustrated in FIG. 1, with layer 12 still in a beaded condition may be migration imaged. This technique may be advantageously employed where the beads are deposited and at least partially embedded, and thereby fixed in a softenable layer previously deposited on the substrate.

Referring now to FIG. 3, the imaging member thus formed is uniformly electrostatically charged, illustratively by means of a corona discharge device 22 which is shown to be traversing the member from left to right depositing a uniform charge, illustratively negative, on the surface of layer 16. For example, corona discharge devices of the general description and generally operated as disclosed in Vyverberg U.S. Pat. No. 2,836,725 and Walkup U.S. Pat. No. 2,777,957 have been found to be excellent sources of corona useful in the charging of member 10. Other charging techniques ranging from rubbing the imate' to induction charging, for example, as described in Walkup U.S. Pat. No. 2,934,649, are available in the art. The surface charge potentials of layer 16 preferred for imaging may be positive or negative and run generally from a few to as high as 4000 volts. Preferably member 10 should be charged in the substantial absence of actinic electromagnetic radia tion for the photosensitive particles 13.

Where substrate 11 is an insulating material, charging of the plate, for example, may be accomplished by placing the insulating substrate in contact with a conductive member and charging as illustrated in FIG. 3. Alternatively, other methods known in the art of xerography for charging xerographic plates having insulating backings may be applied. For example, the member may be charged using double sided corona charging techniques where two corona charging devices on each side of the member and oppositely charged are traversed in register relative to member 10.

FIG. 4 shows a schematic representation of the exposure of the charged imaging member and layer 16 to polychromatic light. For purposes of illustration, each area across the surface of layer 16 is exposed to light of a different color, as follows: area 26 is exposed to red light, area 27 is exposed to blue light, area 28 is exposed to green light, area 29 is exposed to yellow light and area 30 is exposed to white light. For the purposes of illustration, the surface electrical charges deposited in the charging step of FIG. 3 are depicted as having moved into layer 16 in those areas of layer 16 struck by light which they are capable of absorbing. Thus, for example, in area 26, red light is absorbed only by the cyan particles. Magenta and yellow particles which do not absorb the red light retain their surface charge. In area 27 only the yellow particles absorb blue light so that charge remains in magenta and cyan areas. In area 28, only the magenta particles absorb green light, leaving charge in yellow and cyan areas. In area 29, both magenta and cyan particles absorb yellow light so charge is left only in yellow areas. Since all of the particles absorb white light, no surface charge remains in area 10 which was exposed to white light. Thus, exposure leaves a latent image corresponding to the polychromatic exposure.

Although this representation is speculative, it is helpful for an understanding of the present invention to consider electrical charges in light absorbed areas to be no longer residing on the surface of layer 16 and in the light non-absorbed areas to be essentially residing on the surface.

It is not essential that the exposure step result in either the substantial discharge of the member or the appreciable lowering of the electrical fields or surface charge potential in the light absorbed areas. Rather selective relocation of charge into layer 16 is sufficient to produce a developable charge pattern in accordance with the present invention.

If desired, the above described charging and exposure steps may be carried out simultaneously rather than sequentially. Apparatus especially suitable for simultaneous charge/exposure is disclosed, for example, in Gunther et al US. Pat. No. 3,196,0l l. Minimum exposures for use herein generally fall between about and 200 f.c.s.

It will be appreciated that although the latent imaged member is then usually developed after the polychromatic imagewise exposure step, to cause imagewise migration of particles and to render the latent image visible, the latent imaged member is a useful end in itself, being stable for a matter of minutes and thus potentially developable.

Referring now to FIG. 5, the next step, typically, is to develop the latent image to render it visible, which is usually done in the absence of actinic radiation for the member, by softening or dissolving away the softenable material portions of layer 16 to permit portions of the photosensitive particles corresponding to portions of layer 16 with charges still residing thereon after exposure, to migrate in image configuration towards substrate ll. As illustrated, one mode of accomplishing development is liquid solvent developing accomplished by temporarily contacting member 10 with a solvent for softenable material 14, for example by immersing member 10 in container 33 containing a liquid solvent 34 for the softenable material. The softenable material and those photosensitive particles which do not retain a surface charge; that is, those areas which were struck by light which they absorb, are washed away in this bath. Those particles which were not exposed to light which they absorb migrate to the surface of substrate 11, forming a subtractive color image on substrate 11 conforming to the original exposure.

Referring now to FIG. 6, there is shown the final image produced by the inventive process. In area 26, which was exposed to red light, magenta and yellow particles have migrated to the surface of substrate 11, combining to produce a red appearing image. In area 27, which was exposed to blue light, magenta and cyan particles have migrated to substrate 11, combining to form a blue appearing image. In area 28, which was exposed to green light, yellow and cyan particles have migrated to substrate 11, combining to form a green appearing image. In area 29 which was exposed to yellow light, only yellow particles have migrated to substrate 11, forming a yellow image. In area 30, no particles have migrated to the substrate, leaving a white or transparent area corresponding to the color of substrate 11. While as schematically shown in FIG. 6 the areas of different colored particles are slightly spaced, in practice where the different areas are only a few microns in diameter, to the human eye the colors will merge suffciently to give an image appearing to correspond to the original. The particulate image formed on substrate 11 may be fixed by spraying a resin in a solvent thereover, by laminating a clear plastic sheet thereover or by any other suitable technique. Often, not quite all of the softenable matrix material is removed during the development step so that sufficient softenable material remains on the surface of substrate 11 to satisfactorily fix the image forming particles in place. If desired, the image may be transferred from substrate 11 to a receiving sheet. For example, a clear sheet having a pressure sensitive adhesive on the surface thereof may be pressed against substrate 11 to adhesively pick up the image forming particles and transfer them to a receiving sheet, such as paper.

The developer solvent 34 may comprise any suitable liquid in which the softenable material dissolves, while leaving unaffected on the supporting substrate the photosensitive material in the form of the image. The only requirement of the solvent is that it be a solvent for the softenable layer only, and that it be substantially electrically insulating in the sense that the charged image is not discharged electrically by exposure to the solvent before migrating to the substrate. The time of exposure to the solvent is in no way critical, inasmuch as the substrate and photosensitive material are selected so as to be substantially insoluble during development. In gene ral, a few seconds of immersion in the solvent is more than sufficient to dissolve away the softenable material. Typical solvents for use with the various softenable materials 14 include acetone, trichloroethylene, chloroform, ethyl ether, xylene, dioxane, benzene, toluene, cyclohexane. 1,1,1-trichloroethane, pentane, nheptane, Odorless Solvent 3440 (Sohio), trichlorotrifluoroethane, available under the designation Freon 1 13 from DuPont; Freon TMC from DuPont, M xylene, carbon tetrachloride, thiophene, diphenyl ether, pcymene, cis-2,2-dichloroethylene, nitromethane, n,ndimethyl formamide, ethanol, ethyl acetate, methyl ethyl ketone, ethylene dichloride, methylene chloride, trans 1,2-dichloroethylene, Super Naphtholite available from Buffalo Solvents and Chemicals and mixtures thereof.

As described above, with respect to the schematic drawings, the imaging is positive-to-positive since exposed particles, that is, those particles which are exposed to light which they absorb, are washed away and unexposed particles migrate to the substrate. It is also possible to obtain positive-to-negative imaging wherein exposed particles migrate to the substrate while unexposed particles are washed away. For sub tractive color imaging in which natural color is to be reproduced, positive-to-positive imaging is generally preferred. However, in some cases positive-to-negative imaging may be more useful. For example, in the production of posters using two, three or more colors it may be desirable to have exposed particles migrate to the substrate. All of the factors which influence whether a given photosensitive particle will image in the positive-to-positive or positive-to-negative mode are not fully understood. However, it is known that the imaging mode can be influenced by the choice of (I) sign of the surface charge, (2) choice of softenable material, (3) choice of solvent, and (4) choice of photosensitive particle composition. Thus, one should select from the typical photosensitive materials, solvents, and softenable materials listed above, those which will produce images in the desired mode. Techniques for varying the imaging mode are further described in copending applications Ser. Nos. 634,757 and 635,096, filed Apr. 28, 1967, now abandoned and May 1, 1967, respectively.

It should be understood that although the solvent liquid wash-away mode of development is preferred in many instances, because of the quality of the images produced; as described in aforementioned copending applications 460,377 now U.S. Pat. No. 3,520,681 and 483,675 now U.S. Pat. No. 3,656,990 and in copending application Ser. No. 612,122, filed Jan. 27, 1967, now abandoned, development of the imaging members hereof may also be accomplished by softening the softenable layer, for example, with solvent vapor or heat or briefly contacting the softenable layer 16 with a solvent to swell said layer to cause selective imagewise migration of photosensitive particles, and although layer 16 and non-migrated areas of photosensitive particles are not thereby washed away, the image produced may be viewed by means of special display techniques including, for example, focusing light reflected from the member onto a viewing screen. Moreover, a liquid solvent may at any time thereafter be applied to such an image to convert it into a solvent wash-away image as illustrated in FIG. 6. In this regard, it is further noted that the liquid solvent applied in this wash-away step need not be insulating, conductive liquids may be used. It has also been found that nonmigrated background areas of fracturable material of such a migration image may be removed by abrasion to yield a readily visible image, or the illuminated areas may be adhesively stripped off to yield complementary positive and negative images.

The following Examples describe specific embodiments and methods of producing polychromatic images using the system of this invention. Parts and percentages are by weight unless otherwise indicated. The following Examples should be considered as describing preferred embodiments of the system of this invention.

EXAMPLE 1 Three photosensitive particle-softenable material dispersions are prepared as follows: About 2 parts Monolite Fast Blue GS, a mixture of alpha and beta form metal-free phthalocyanine, available from Arnold Hoffman Co. is dispersed in a solution of about 6 parts Piccotex 100 in about 50 parts lsopar G, a long chain saturated aliphatic hydrocarbon liquid boiling point 315-350F. from Humble Oil Co. The second dispersion is prepared by dispersing about 2 parts of lrgazin Red-ZBLT, as described in U.S. Pat. No. 2,973,358, available from Geigy Chemical Corp. in a solution of about 6 parts Piccotex 100 in about 50 parts lsopar G. The third dispersion is prepared by dispersing about 2 parts of a finely divided yellow pigment, Algol Yellow GC, C.l. No. 67300, l,2,5,6-di(C, C'-diphenyl)- thiazole-anthraquinone, available from General Dye Stuffs, in a solution of about 6 parts Piccotex 100 in about 50 parts lsopar G. Each of these dispersions is separately spray dried as by the process described in copending application 380,080, filed June 2, 1964, now U.S. Pat. No. 3,338,991 forming beads of Piccotex 100 softenable material having the photosensitive pigment particles uniformly distributed therethrough. All of the beads are then uniformly mixed together. A uniform, substantial monolayer of the bead mixture is then formed on an aluminum substrate by the powder cloud loading process described in Gundlach U.S. Pat. No.

3,166,432. A heated metal roller having a thin layer of polytetrafluoroethylene on the surface is then rolled across the beads. The roller is maintained at a temperature of about C. which softens the beads as the roller passes over the plate. A uniform smooth surfaced layer is thus formed, having discrete areas containing pigment particles of each color.

The thus prepared plate is then uniformly electrostatically charged to a negative potential of about 200 volts by corona discharge as described in Carlson U.S. Pat. No. 2,588,699. The plate is then exposed to a color original using a conventional Kodachrome transparency. Total exposure is about 260 f.c.s. in illuminated areas. The exposed plate is then developed by dipping it in a container containing trichloroethylene. After a few seconds in the solvent, the plate is removed. An image consisting of migrated particles is seen on the plate conforming to the original. Unneeded particles and most of the resin is removed by the solvent.

EXAMPLE [1 Example 1 is followed except that the cyan pigment is the X-form of metal-free phthalocyanine, as described in copending application Ser. No. 505,723, filed Oct. 29, 1965 now U.S. Pat. No. 3,357,989.

EXAMPLE 111 Example 1 is followed except that the magenta pigment is Watchung Red B, C. 1. No. 15865, from Du- Pont.

EXAMPLE 1V Example 1 is followed except that the magenta pigment is 2-(4'-toluazo)-4-isopropoxy-l-naphthol prepared as described in copending application Serial No. 445,240, filed Apr. 2, 1965, now U.S. Pat. No. 3,384,632, and the yellow photosensitive particles are N-2"-pyridyl-8,13-dioxodinaphtho-( l ,2-2',3')-furan- 6-carboxamide, prepared as described in copending application Ser. No. 421,281, filed Dec. 28, 1964 now U.S. Pat. No. 3,447,992.

EXAMPLE V Example 1 is followed except that the magenta pigment is Naphthol Red B and the cyan colored pigment is Diana Blue.

Although specific components and proportions have been stated in the above description of preferred embodiments of the polychromatic migration imaging system hereof, other suitable materials as listed herein may be used with similar results. In addition, other materials may be added to materials used herein or variations may be made in the various processing steps hereof to synergize, enhance or otherwise modify the invention. For example, the photosensitive materials and the softenable materials may be electrically or dye sensitized to narrow, broaden or heighten their spectral response curves. Also, plasticizers, moisture and other proofing agents may be added to the softenable materials hereof, as desired.

It will be understood that various other changes in the details, materials, steps and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention will occur to and may be made by those skilled in the art upon a reading of this disclosure, and such changes are intended to be included within the principle and scope of this invention. I

What is claimed is:

l. A color imaging method comprising the steps of:

a. providing an imaging member comprising a supporting substrate, said substrate impermeable to electrically photosensitive particles under the development conditions of step (d) below, an overlayer contacting said substrate consisting essentially of a mosaic pattern having a plurality of contiguous beads of different colors forming a monolayer, each bead comprising electrically photosensitive particles of single color dispersed throughout 7 an electrically insulating softenable material, said softenable material capable of having its resistance to migration of said electrically photosensitive particles decreased sufficiently to allow migration of said electrically photosensitive material through said softenable material toward said substrate;

b. substantially uniformly electrostatically charging said monolayer of beads;

c. exposing said monolayer of beads to an image pattern of polychromatic light; and

d. developing said member by decreasing the resistance of the softenable material contained in each bead to migration of said electrically photosensitive particles contained in each bead through the softenable material at least sufficient to allow imagewise migration of electrically photosensitive particles through the softenable material toward but not into the substrate.

2. The color imaging method according to claim 1 wherein said developing is accomplished by softening said softenable material with an electrically insulating solvent for said softenable material.

3. The color imaging method according to claim 1 wherein said developing is accomplished by dissolving said softenable material with an electrically insulating solvent for said softenable material.

4. The color imaging method according to claim 3 wherein the solvent washes away said softenable material and portions of said photosensitive particles, leaving on said substrate, portions of said particles in a pattern corresponding to said image pattern.

5. The color imaging method according to claim 1 wherein said developing is accomplished by softening said softenable material by subjecting it to heat.

6. The color imaging method according to claim 1 wherein said developing is accomplished by softening said softenable material by subjecting it to the vapors of an electrically insulating solvent for said softenable material.

7. The color imaging method of claim 1 wherein said particles comprise cyan colored particles, sensitive mainly to red light, magenta colored particles, sensitive mainly to green light and yellow colored particles, sensitive mainly to blue light.

8. The color imaging method of claim 1 wherein said layer has a thickness of from about 0.5 to about 16 microns.

'9. The color imaging method of claim 8 wherein said layer has a thickness of from about 1 to about 4 microns.

10. The color imaging method of claim 1 wherein said photosensitive particles have an average diameter of from about 0.02 to about 2.0 microns.

11. The color imaging method of claim 10 wherein said photosensitive particles have an average diameter of from about 0.03 to about 0.5 micron.

12. The method of claim 1 wherein the photosensitive particles exposed to said light migrate toward said substrate.

13. The method of claim 1 wherein the photosensitive particles not exposed to light migrate toward said substrate.

14. The method of claim 1 wherein said mosaic pattern layer is formed by converting the monolayer of beads comprising electrically insulating softenable material containing the photosensitive particles dispersed throughout said softenable material into a smooth surfaced layer.

15. A method according to claim 13 wherein said layer of beads is converted into a smooth layer by pressing said layer of beads with a member heated to substantially the softenable temperature of said softenable material.

16. A method according to claim 13 wherein said layer of beads is converted into a smooth layer by heating said layer of beads to the fusing temperature of said softenable material and permitting surface tension forces to smooth the surface of said layer, and cooling said layer below the softening temperature of said softenable material.

17. A method according to claim 14 wherein said layer of beads is converted into a smooth layer by subjecting said layer of beads to the vapor of a solvent for said softenable material and permitting surface tension forces to smooth the surface of said layer. 7

18. A method according to claim 1 wherein said layer of beads is formed by triboelectrically charging the beads and carrying them into contact with a substrate in an air stream whereby substantially a monolayer of said beads is formed on said substrate by electrostatic attraction.

19. The method of claim 1 wherein the number of beads of each color are substantially equal and are of substantially the same size.

20. The method of claim 1 wherein there are three intermixed sets of beads, one set containing cyan colored photosensitive particles sensitive mainly to red light, a second set containing magenta colored photosensitive particles sensitive mainly to green light and a third set containing yellow colored photosensitive particles sensitive mainly to blue light.

21. A method according to claim 18 wherein said beads are from about I to about 30 microns in average diameter and wherein said photosensitive particles are from about 0.02 to about 2 microns in average diameter.

22. A method according to claim 21 wherein said photosensitive particles are from about 0.03 to about 0.5 microns in average diameter.

23. A method according to claim 20 including the step of converting said monolayer of beads into a smooth surfaced layer adhering to said substrate before said charging step.

24'.'"A method according to claim 23 wherein said layer of beads is converted into a smooth layer by pressing said layer of beads with a member heated to substantially the softening temperature of said softenable material.

25. A method according to claim 23 wherein said layer of beads is converted into a smooth layer by heatforces to smooth the surface of said layer.

27. A method according to claim 20 wherein said layer of beads is formed by triboelectrically charging the beads and carrying them into contact with a substrate in an air stream whereby substantially a monolayer of said beads is formed on said substrate by electrostatic attraction. 

1. A COLOR IMAGINING METHOD COMPRISING THE STEPS OF: A. PROVIDING AN IMAGING MEMBER COMPRISING A SUPPORTING SUBSTRATE, SAID SUBSTRATE IMPERMEABLE TO ELECTRICALLY PHOTOSENSITIVE PARTICLES UNDER THE DEVELOPMENT CONDITIONS OF STEP (D) BELOW, AN OVERLAYER CONTACTING SAID SUBSTRATE CONSISTING ESSENTIALLY OF A MOSAIC PATTERN HAVING A PLURALITY OF CONTIGUOUS BEADS OF DIFFERENT COLORS FORMING A MONOLAYER, EACH BEAD COMPRISING ELECTRICALLY PHOTOSENSITIVE PARTICLES OF SINGLE COLOR DISPERSED THROUGHOUT AN ELECTRICALLY INSULATING SOFTENABLE MATERIAL, SAID SOFTENABLE MATERIAL CAPABLE OF HAVING ITS RESISTANCE TO MIGRATION OF SAID ELECTRICALLY PHOTOSENSITIVE PARTICLES DECREASED SUFFICIENTLY TO ALLOW MIGRATION OF SAID ELECTRICALLY PHOTOSENSITIVE MATERIAL THROUGHOUT SAID SOFTENABLE MATERIAL TOWARD SAID SUBSTRATE, B. SUBSTANTIALLY UNIFORMLY ELECTROSTATICALLY CHARGING SAID MONOLAYER OF BEADS, C. EXPOSING SAID MONOLAYER OF BEADS TO AN IMAGE PATTERN OF POLYCHROMATIC LIGHT, AND D. DEVELOPING SAID MEMBER BY DECREASING THE RESISTANCE OF THE SOFTENABLE MATERIAL CONTAINED IN EACH BEAD TO MIGRATION OF SAID ELECTRICALLY PHOTOSENSITIVE PARTICLES CONTAINED IN EACH BEAD THROUGH THE SOFTENABLE MATERIAL AT LEAST SUFFICIENT TO ALLOW IMAGEWISE MIGRATION OF ELECTRICALLY PHOTOSENSITIVE PARTICLES THROUGH THE SOFTENABLE MATERIAL TOWARD BUT NOT INTO THE SUBSTRATE.
 2. The color imaging method according to claim 1 wherein said developing is accomplished by softening said softenable material with an electrically insulating solvent for said softenable material.
 3. The color imaging method according to claim 1 wherein said developing is accomplished by dissolving said softenable material with an electrically insulating solvent for said softenable material.
 4. The color imaging method according to claim 3 wherein the solvent washes away said softenable material and portions of said photosensitive particles, leaving on said substrate, portions of said particles in a pattern corresponding to said image pattern.
 5. The color imaging method according to claim 1 wherein said developing is accomplished by softening said softenable material by subjecting it to heat.
 6. The color imaging method according to claim 1 wherein said developing is accomplished by softening said softenable material by subjecting it to the vapors of an electrically insulating solvent for said softenable material.
 7. The color imaging method of claim 1 wherein said particles comprise cyan colored particles, sensitive mainly to red light, magenta colored particles, sensitive mainly to green light and yellow colored particles, sensitive mainly to blue light.
 8. The color imaging method of claim 1 wherein said layer has a thickness of from about 0.5 to about 16 microns.
 9. The color imaging method of claim 8 wherein said layer has a thickness of from about 1 to about 4 microns.
 10. The color imaging method of claim 1 wherein said photosensitive particles have an average diameter of from about 0.02 to about 2.0 microns.
 11. The color imaging method of claim 10 wherein said photosensitive particles have an average diameter of from about 0.03 to about 0.5 micron.
 12. The method of claim 1 wherein the photosensitive particles exposed to said light migrate toward said substrate.
 13. The method of claim 1 wherein the photosensitive particles not exposed to light migrate toward said substrate.
 14. The method of claim 1 wherein said mosaic pattern layer is formed by converting the monolayer of beads comprising electrically insulating softenable material containing the photosensitive particles dispersed throughout said softenable material into a smooth surfaced layer.
 15. A method according to claim 13 wherein said layer of beads is converted into a smooth layer by pressing said layer of beads with a member heated to substantially the softenable temperature of said softenable material.
 16. A method according to claim 13 wherein said layer of beads is converted into a smooth layer by heating said layer of beads to the fusing temperature of said softenable material and permitting surface tension forces to smooth the surface of said layer, and cooling said layer below the softening temperature of said softenable material.
 17. A method according to claim 14 wherein said layer of beads is converted into a smooth layer by subjecting said layer of beads to the vapor of a solvent for said softenable material and permitting surface tension forces to smooth the surface of said layer.
 18. A method according to claim 1 wherein said layer of beads is formed by triboelectrically charging the beads and carrying them into contact with a substrate in an air stream whereby substantially a monolayer of said beads is formed on said substrate by electrostatic attraction.
 19. The method of claim 1 wherein the number of beads of each color are substantially equal and are of substantially the same size.
 20. The method of claim 1 wherein there are three intermixed sets of beads, one set containing cyan colored photosensitive particles sensitive mainly to red light, a second set containing magenta colored photosensitive particles sensitive mainly to green light and a third set containing yellow colored photosensitive particles sensitive mainly to blue light.
 21. A method according to claim 18 wherein said beads are from about 1 to about 30 microns in average diameter and wherein said photosensitive particles are from about 0.02 to about 2 microns in average diameter.
 22. A method according to claim 21 wherein said photosensitive particles are from about 0.03 to about 0.5 microns in average diameter.
 23. A method according to claim 20 including the step of converting said monolayer of beads into a smooth surfaced layer adhering to said substrate before said charging step.
 24. A method according to claim 23 wherein said layer of beads is converted into a smooth layer by pressing said layer of beads with a member heated to substantially the softening temperature of said softenable material.
 25. A method according to claim 23 wherein said layer of beads is converted into a smooth layer by heating said layer of beads to the fusing temperature of said softenable material and permitting surface tension forces to smooth the surface of said layer, and cooling said layer below the softening temperature of said softenable material.
 26. A method according to claim 23 wherein said layer of beads is converted into a smooth layer by subjecting said layer of beads to the vapor of a solvent for said softenable material and permitting surface tension forces to smooth the surface of said layer.
 27. A method according to claim 20 wherein said layer of beads is formed by triboelectrically charging the beads and carrying them into contact with a substrate in an air stream whereby substantially a monolayer of said beads is formed on said substrate by electrostatic attraction. 